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Subject[PATCH AUTOSEL 4.4 55/56] pwm: Fix deadlock warning when removing PWM device
Date
From: Phong Hoang <phong.hoang.wz@renesas.com>

[ Upstream commit 347ab9480313737c0f1aaa08e8f2e1a791235535 ]

This patch fixes deadlock warning if removing PWM device
when CONFIG_PROVE_LOCKING is enabled.

This issue can be reproceduced by the following steps on
the R-Car H3 Salvator-X board if the backlight is disabled:

# cd /sys/class/pwm/pwmchip0
# echo 0 > export
# ls
device export npwm power pwm0 subsystem uevent unexport
# cd device/driver
# ls
bind e6e31000.pwm uevent unbind
# echo e6e31000.pwm > unbind

[ 87.659974] ======================================================
[ 87.666149] WARNING: possible circular locking dependency detected
[ 87.672327] 5.0.0 #7 Not tainted
[ 87.675549] ------------------------------------------------------
[ 87.681723] bash/2986 is trying to acquire lock:
[ 87.686337] 000000005ea0e178 (kn->count#58){++++}, at: kernfs_remove_by_name_ns+0x50/0xa0
[ 87.694528]
[ 87.694528] but task is already holding lock:
[ 87.700353] 000000006313b17c (pwm_lock){+.+.}, at: pwmchip_remove+0x28/0x13c
[ 87.707405]
[ 87.707405] which lock already depends on the new lock.
[ 87.707405]
[ 87.715574]
[ 87.715574] the existing dependency chain (in reverse order) is:
[ 87.723048]
[ 87.723048] -> #1 (pwm_lock){+.+.}:
[ 87.728017] __mutex_lock+0x70/0x7e4
[ 87.732108] mutex_lock_nested+0x1c/0x24
[ 87.736547] pwm_request_from_chip.part.6+0x34/0x74
[ 87.741940] pwm_request_from_chip+0x20/0x40
[ 87.746725] export_store+0x6c/0x1f4
[ 87.750820] dev_attr_store+0x18/0x28
[ 87.754998] sysfs_kf_write+0x54/0x64
[ 87.759175] kernfs_fop_write+0xe4/0x1e8
[ 87.763615] __vfs_write+0x40/0x184
[ 87.767619] vfs_write+0xa8/0x19c
[ 87.771448] ksys_write+0x58/0xbc
[ 87.775278] __arm64_sys_write+0x18/0x20
[ 87.779721] el0_svc_common+0xd0/0x124
[ 87.783986] el0_svc_compat_handler+0x1c/0x24
[ 87.788858] el0_svc_compat+0x8/0x18
[ 87.792947]
[ 87.792947] -> #0 (kn->count#58){++++}:
[ 87.798260] lock_acquire+0xc4/0x22c
[ 87.802353] __kernfs_remove+0x258/0x2c4
[ 87.806790] kernfs_remove_by_name_ns+0x50/0xa0
[ 87.811836] remove_files.isra.1+0x38/0x78
[ 87.816447] sysfs_remove_group+0x48/0x98
[ 87.820971] sysfs_remove_groups+0x34/0x4c
[ 87.825583] device_remove_attrs+0x6c/0x7c
[ 87.830197] device_del+0x11c/0x33c
[ 87.834201] device_unregister+0x14/0x2c
[ 87.838638] pwmchip_sysfs_unexport+0x40/0x4c
[ 87.843509] pwmchip_remove+0xf4/0x13c
[ 87.847773] rcar_pwm_remove+0x28/0x34
[ 87.852039] platform_drv_remove+0x24/0x64
[ 87.856651] device_release_driver_internal+0x18c/0x21c
[ 87.862391] device_release_driver+0x14/0x1c
[ 87.867175] unbind_store+0xe0/0x124
[ 87.871265] drv_attr_store+0x20/0x30
[ 87.875442] sysfs_kf_write+0x54/0x64
[ 87.879618] kernfs_fop_write+0xe4/0x1e8
[ 87.884055] __vfs_write+0x40/0x184
[ 87.888057] vfs_write+0xa8/0x19c
[ 87.891887] ksys_write+0x58/0xbc
[ 87.895716] __arm64_sys_write+0x18/0x20
[ 87.900154] el0_svc_common+0xd0/0x124
[ 87.904417] el0_svc_compat_handler+0x1c/0x24
[ 87.909289] el0_svc_compat+0x8/0x18
[ 87.913378]
[ 87.913378] other info that might help us debug this:
[ 87.913378]
[ 87.921374] Possible unsafe locking scenario:
[ 87.921374]
[ 87.927286] CPU0 CPU1
[ 87.931808] ---- ----
[ 87.936331] lock(pwm_lock);
[ 87.939293] lock(kn->count#58);
[ 87.945120] lock(pwm_lock);
[ 87.950599] lock(kn->count#58);
[ 87.953908]
[ 87.953908] *** DEADLOCK ***
[ 87.953908]
[ 87.959821] 4 locks held by bash/2986:
[ 87.963563] #0: 00000000ace7bc30 (sb_writers#6){.+.+}, at: vfs_write+0x188/0x19c
[ 87.971044] #1: 00000000287991b2 (&of->mutex){+.+.}, at: kernfs_fop_write+0xb4/0x1e8
[ 87.978872] #2: 00000000f739d016 (&dev->mutex){....}, at: device_release_driver_internal+0x40/0x21c
[ 87.988001] #3: 000000006313b17c (pwm_lock){+.+.}, at: pwmchip_remove+0x28/0x13c
[ 87.995481]
[ 87.995481] stack backtrace:
[ 87.999836] CPU: 0 PID: 2986 Comm: bash Not tainted 5.0.0 #7
[ 88.005489] Hardware name: Renesas Salvator-X board based on r8a7795 ES1.x (DT)
[ 88.012791] Call trace:
[ 88.015235] dump_backtrace+0x0/0x190
[ 88.018891] show_stack+0x14/0x1c
[ 88.022204] dump_stack+0xb0/0xec
[ 88.025514] print_circular_bug.isra.32+0x1d0/0x2e0
[ 88.030385] __lock_acquire+0x1318/0x1864
[ 88.034388] lock_acquire+0xc4/0x22c
[ 88.037958] __kernfs_remove+0x258/0x2c4
[ 88.041874] kernfs_remove_by_name_ns+0x50/0xa0
[ 88.046398] remove_files.isra.1+0x38/0x78
[ 88.050487] sysfs_remove_group+0x48/0x98
[ 88.054490] sysfs_remove_groups+0x34/0x4c
[ 88.058580] device_remove_attrs+0x6c/0x7c
[ 88.062671] device_del+0x11c/0x33c
[ 88.066154] device_unregister+0x14/0x2c
[ 88.070070] pwmchip_sysfs_unexport+0x40/0x4c
[ 88.074421] pwmchip_remove+0xf4/0x13c
[ 88.078163] rcar_pwm_remove+0x28/0x34
[ 88.081906] platform_drv_remove+0x24/0x64
[ 88.085996] device_release_driver_internal+0x18c/0x21c
[ 88.091215] device_release_driver+0x14/0x1c
[ 88.095478] unbind_store+0xe0/0x124
[ 88.099048] drv_attr_store+0x20/0x30
[ 88.102704] sysfs_kf_write+0x54/0x64
[ 88.106359] kernfs_fop_write+0xe4/0x1e8
[ 88.110275] __vfs_write+0x40/0x184
[ 88.113757] vfs_write+0xa8/0x19c
[ 88.117065] ksys_write+0x58/0xbc
[ 88.120374] __arm64_sys_write+0x18/0x20
[ 88.124291] el0_svc_common+0xd0/0x124
[ 88.128034] el0_svc_compat_handler+0x1c/0x24
[ 88.132384] el0_svc_compat+0x8/0x18

The sysfs unexport in pwmchip_remove() is completely asymmetric
to what we do in pwmchip_add_with_polarity() and commit 0733424c9ba9
("pwm: Unexport children before chip removal") is a strong indication
that this was wrong to begin with. We should just move
pwmchip_sysfs_unexport() where it belongs, which is right after
pwmchip_sysfs_unexport_children(). In that case, we do not need
separate functions anymore either.

We also really want to remove sysfs irrespective of whether or not
the chip will be removed as a result of pwmchip_remove(). We can only
assume that the driver will be gone after that, so we shouldn't leave
any dangling sysfs files around.

This warning disappears if we move pwmchip_sysfs_unexport() to
the top of pwmchip_remove(), pwmchip_sysfs_unexport_children().
That way it is also outside of the pwm_lock section, which indeed
doesn't seem to be needed.

Moving the pwmchip_sysfs_export() call outside of that section also
seems fine and it'd be perfectly symmetric with pwmchip_remove() again.

So, this patch fixes them.

Signed-off-by: Phong Hoang <phong.hoang.wz@renesas.com>
[shimoda: revise the commit log and code]
Fixes: 76abbdde2d95 ("pwm: Add sysfs interface")
Fixes: 0733424c9ba9 ("pwm: Unexport children before chip removal")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Hoan Nguyen An <na-hoan@jinso.co.jp>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/pwm/core.c | 10 +++++-----
drivers/pwm/sysfs.c | 14 +-------------
include/linux/pwm.h | 5 -----
3 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/drivers/pwm/core.c b/drivers/pwm/core.c
index ec84ff8ad1b4a..6911f96623001 100644
--- a/drivers/pwm/core.c
+++ b/drivers/pwm/core.c
@@ -284,10 +284,12 @@ int pwmchip_add_with_polarity(struct pwm_chip *chip,
if (IS_ENABLED(CONFIG_OF))
of_pwmchip_add(chip);

- pwmchip_sysfs_export(chip);
-
out:
mutex_unlock(&pwm_lock);
+
+ if (!ret)
+ pwmchip_sysfs_export(chip);
+
return ret;
}
EXPORT_SYMBOL_GPL(pwmchip_add_with_polarity);
@@ -321,7 +323,7 @@ int pwmchip_remove(struct pwm_chip *chip)
unsigned int i;
int ret = 0;

- pwmchip_sysfs_unexport_children(chip);
+ pwmchip_sysfs_unexport(chip);

mutex_lock(&pwm_lock);

@@ -341,8 +343,6 @@ int pwmchip_remove(struct pwm_chip *chip)

free_pwms(chip);

- pwmchip_sysfs_unexport(chip);
-
out:
mutex_unlock(&pwm_lock);
return ret;
diff --git a/drivers/pwm/sysfs.c b/drivers/pwm/sysfs.c
index 375008e2be20a..199370e41da90 100644
--- a/drivers/pwm/sysfs.c
+++ b/drivers/pwm/sysfs.c
@@ -338,19 +338,6 @@ void pwmchip_sysfs_export(struct pwm_chip *chip)
}

void pwmchip_sysfs_unexport(struct pwm_chip *chip)
-{
- struct device *parent;
-
- parent = class_find_device(&pwm_class, NULL, chip,
- pwmchip_sysfs_match);
- if (parent) {
- /* for class_find_device() */
- put_device(parent);
- device_unregister(parent);
- }
-}
-
-void pwmchip_sysfs_unexport_children(struct pwm_chip *chip)
{
struct device *parent;
unsigned int i;
@@ -368,6 +355,7 @@ void pwmchip_sysfs_unexport_children(struct pwm_chip *chip)
}

put_device(parent);
+ device_unregister(parent);
}

static int __init pwm_sysfs_init(void)
diff --git a/include/linux/pwm.h b/include/linux/pwm.h
index aa8736d5b2f38..cfc3ed46cad20 100644
--- a/include/linux/pwm.h
+++ b/include/linux/pwm.h
@@ -331,7 +331,6 @@ static inline void pwm_remove_table(struct pwm_lookup *table, size_t num)
#ifdef CONFIG_PWM_SYSFS
void pwmchip_sysfs_export(struct pwm_chip *chip);
void pwmchip_sysfs_unexport(struct pwm_chip *chip);
-void pwmchip_sysfs_unexport_children(struct pwm_chip *chip);
#else
static inline void pwmchip_sysfs_export(struct pwm_chip *chip)
{
@@ -340,10 +339,6 @@ static inline void pwmchip_sysfs_export(struct pwm_chip *chip)
static inline void pwmchip_sysfs_unexport(struct pwm_chip *chip)
{
}
-
-static inline void pwmchip_sysfs_unexport_children(struct pwm_chip *chip)
-{
-}
#endif /* CONFIG_PWM_SYSFS */

#endif /* __LINUX_PWM_H */
--
2.20.1
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